Lithium metal has many uses and, to name a few, they include nuclear power application where a blanket of the liquid metal or its molten salts is used for breeding purposes in nuclear fusion reactors, in lightweight, compact lithium/sulfur batteries for electric cars and for power plant load leveling purposes, as a degasifier in the production of high-conductivity copper and bronze, and in the synthesis of compounds for use in the field of medicine.
Lithium metal is generally produced by electrolysis of eutectic mixture of highly pure molten lithium chloride and potassium chloride.
There are naturally occurring brines in the United States which contain reasonable concentrations of lithium, in the form of the chloride, so as to be considered viable reserves for lithium recovery. Three particular sources include Searle's Lake, California, the Great Salt Lake, Utah, and Clayton Valley, Nevada. The latter is the most economical source of lithium since the magnesium to lithium ratio is low, generally about 1.15:1, which allows for a simplified process of concentrating, purifying and recovering lithium chloride brine. Lithium carbonate is then obtained by treatment of the brine with soda ash.
To make lithium metal, the lithium carbonate is converted to lithium hydroxide via a liming process, and the latter compound in turn is converted to lithium chloride by treatment with hydrochloric acid following by drying. This is a very circuitous and expensive route to lithium chloride, since lithium originally exists as the chloride in the natural brine. Thus, for many years there has been the need for a direct economical method for recovering lithium chloride as such from natural brines.
As noted, natural brine typically contains only a few hundred parts per million of lithium in conjunction with substantial quantities of sodium, potassium and magnesium chlorides and sulfates, as well as other minor contaminants such as bromides, rubidium, boron and organic compounds. In the manufacture of lithium metal by electrolysis of lithium chloride, the alkaline earth metals must first be removed from the lithium chloride, otherwise they will be present as contaminants in the lithium metal. Similarly, sodium must be removed since the presence of very small quantities thereof in the lithium metal will make it highly reactive and much different in properties than high purity lithium metal. Also, during the electrolysis of lithium chloride, non-volatile anions, such as sulfate and borate, will build up resulting in very rapid short circuiting of the cell. Since cell costs are quite high, continuous uninterrupted operation thereof for extended periods of time, e.g. six months to a year, must be realized.
It is a primary object of this invention to provide an improved process for producing lithium chloride of high purity.
Another object of this invention is to provide a novel method for treating lithium chloride to improve the filterability of solvent solutions thereof whereby the amounts of such contaminants as sodium, alkaline earth metals, boron and organics may be reduced to such levels as to render the lithium chloride suitable for use as an electrolyte in the production of high purity lithium metal.
Other objects and advantages of the present invention will become apparent from the following description of specific embodiments thereof and the appended claims.